Thymoma & Thymic Carcinoma: The Basics

Charles Wood, MDUpdated By: Lara Bonner Millar, MD The Abramson Cancer Center of the University of Pennsylvania Last Modified: June 1, 2011

What is thymoma?

Thymoma is a rare tumor of the thymus gland, which is a small organ that lies under the breastbone, in a part of the body known as the "anterior mediastinum." The thymus is part of the immune system and is responsible for the development of lymphocytes, which are cells that travel through the body and help to fight infection. As we age, the thymus regresses, but in some people a remnant remains in adulthood. Thymoma is usually a slow-growing tumor that does not typically spread beyond the thymus gland. It is the most common tumor seen in the anterior mediastinum in adults.

Thymic carcinoma, on the other hand, is less common but more aggressive. It is generally more difficult to treat because it tends to spread quickly to other areas of the body.

Who gets thymoma?

The cause of thymoma is unknown, and the risk factors have not been identified. It affects men and women equally, most often in the fifth and sixth decades of life. People with thymoma may have other diseases of the immune system. Myasthenia gravis, an immune condition that causes the muscles to become weak, is the syndrome most often associated with thymoma, and is present in about 30% of people with thymoma. Conversely, because myasthenia gravis is more common than thymoma, only 10-15% of patients diagnosed with myasthenia gravis also have thymoma.

So what are the symptoms?

One-third to one-half of patients may have no symptoms at all. In such cases, the thymoma is usually discovered on a chest x-ray or a computed tomographic (CT) scan done for another reason. About one-third of patients may present with symptoms related to disease in the chest, such as cough, chest pain, shortness of breath, hoarseness and trouble swallowing. Finally, the last one-third can present with systemic symptoms related to myasthenia gravis, such as muscle weakness.

How is it diagnosed?

Thymoma is usually diagnosed based on x-ray and images of the chest. Laboratory studies such as routine bloodwork are generally not used. If x-ray or CT images reveal the presence of a thymoma with uncommon features, or if there is a question of invasion into other nearby structures in the chest, it may be necessary to obtain a tissue sample of the mass, or biopsy, for examination under a microscope. The biopsy is performed either by insertion of a needle through the chest wall or by a more invasive surgical procedure under general anesthesia, in which an incision is made above the breastbone and a piece of the tumor is removed. This procedure is sometimes done with the aid of a small camera, or scope, inserted into the chest (called video-assisted thoracoscopy, or VATS).

How is thymoma staged?

Once a thymoma is found, it may be necessary to perform more tests to see if the tumor has spread and so that the appropriate treatment can be recommended. The extent of the tumor spread is also referred to as the "stage". The Masaoka staging system is the most commonly used system to stage thymoma. (Note that the staging system for thymoma is also used for thymic carcinoma)

Stage I

The tumor involves only the thymus gland and has not invaded the capsule that surrounds the gland. This is also referred to as noninvasive thymoma.

Stage II

The tumor has spread beyond the thymus gland to invade the capsule that surrounds the gland, the fat that surrounds the gland, or the lining of the lung cavity. Stages II and higher are also referred to as invasive thymoma.

Stage III

The tumor has invaded nearby organs, such as the lungs, the lining of the heart, or the large vessels next to the heart.

Stage IV

The tumor has spread deeper throughout the lining of the heart or the lining of the lungs (stage IVa), or has spread to other areas of the body through the bloodstream or lymphatic system (stage IVb).

Recurrent

The tumor has come back (recurred) after it was originally treated. Most recurrences occur close to where the tumor was originally.

What is the prognosis of the tumor?

The prognosis is based largely on the stage of disease. In the above staging system, the 5-year survival rates are:

96% for stage I

86% for stage II

69% for stage III

50% for stage IV

A more recent but less widely used staging system was introduced by the World Health Organization (WHO). This system stages the disease based on how the cells appear under the microscope. The most favorable tumors are medullary and mixed thymomas (100% survival at 10 years); the next most favorable are cortical and predominantly cortical tumors (83% survival at 10 years); and the least favorable are thymic carcinomas (28-36% survival at 10 years).

What are the recommended treatment options?

All patients who are able to have their tumor completely surgically removed should undergo surgery. Complete removal of the tumor is the best chance for long-term survival. Stage I and II tumors, and some stage III tumors, are generally able to be completely resected.

Radiation Therapy

Thymomas are considered to be sensitive to radiation therapy treatment (which is the use of high- energy x-rays aimed at the tumor or area from where the tumor was removed).

There is no need to use radiation for completely removed noninvasive thymoma tumors (stage I), but radiation is nearly always used in stage III or IV tumors after complete or partial (known as "debulking") surgical removal of the tumor. Whether radiation is needed after complete surgical removal of stage II tumors is controversial, although radiation is often recommended in order to decrease the chance that the tumor will come back in the original site. Additionally, radiation may be used in any stage of thymoma where it is not technically possible to do a safe and complete surgery.

Chemotherapy

The use of chemotherapy for advanced stage tumors has increased over the last decade, and the most commonly used drugs are cisplatin, doxorubicin, and ifosfamide. Corticosteroids are non-chemotherapy drugs that are also sometimes used. The use of more than one drug at a time (combination chemotherapy), either before or after treatment, such as surgery and/or radiation therapy, has been shown to be more effective than the use of only one chemotherapy drug alone.

Stage I

Treatment may be one of the following:

Complete surgical removal of the tumor

Stage II

Treatment may be one of the following:

Complete surgical removal of the tumor

+/- Radiation after surgery

Stage III and IV and Recurrent Disease

Treatment may be one of the following:

Complete surgical removal of the tumor if possible

Radiation after surgery

Radiation alone if the tumor is considered unresectable (unable to be completely removed)

Chemotherapy before and/or after surgery and/or radiation, especially if the tumor is considered unresectable (unable to be completely removed) before surgery

Chemotherapy alone, especially if the cancer has spread widely throughout the body, and there are no significant symptoms associated with the tumor in the chest